Self discovered solution.

Some time back (long enough for me to forget doing so) I transferred SkyDemon to the SD card. The program runs from that location. The original files were retained on the hard drive and it was those I was placing my User Documents into, hence why I couldn't see them. 

Placing the charts into the equivalent folder on the SD card has resumed normal ops. 

Mods. Delete this thread if desired. 

For some time I've been using the UD facility to store .pdf copies of the Jeppesen charts for my home field and primary diversion. This has worked fine on a Lenovo tablet with the relevant folders stored in the (Android > Data > aero.skydemon.skydemonandroid > Files). folder.